Dynamic Interaction between Toc159 and Blue Light Receptors at the Chloroplast Outer Envelope of Pisum sativum (L.)

Physiological significance of Pisum sativum TOC GTPases protein phosphoryation has linked the regulation of protein import into chloroplast with the environmental changes. Phototropin 2 is localized at outer membrane of chloroplast. It was crucial to study the relation between Phot2 localization at chloroplast membrane and phosphorylation of TOC GTPases under blue light intensities. Kinase activity of Phot2 towards TOC GTPases was characterized in outer membrane of Pisum sativum. TOC GTPases were specifically and differentially phosphorylated under diffrent light quality and intensities. Gene chip analysis was performed to correlate between expression of TOC complex subunits and blue light receptors genes using A. thaliana. It was found that transcript level of AtToc33 was the highest observed expression with AtToc159 and AtToc75-III. Also, 26 phototropins and photropin-like genes were identified. Coexpression gene netweork of AtPhot2 has revealed a close connection with TPR-containing protein (At1g01320) and chloroplast Toc159 (AT4G15810). Co-immunoprecipitaion assay has evidenced that no intercation was detected between AtPhot2-KD and PsToc34. However, AtPhot2-KD was found to interact with AtToc159. Chloroplast outer and inner envelopes purified from A. thaliana knockout line of Phot2 has phosphorylated AtToc33 and AtToc86/159 in vitro. Hereby, Phot2 is not the specific protein kinase for TOC GTPases. Hence, it was suggested that certain signal cascades may directly or indirectly link phosphorylation of TOC GTPases, protein levels of PHOT-LIKE proteins and irradiation conditions. Therefore, TOC GTPases phosphorylation might be an external regulatory signal to regulate preproteins import into chloroplasts in response to light changes or as a signal of chloroplast biogenesis.